Generally, a transmission system of an Orthogonal Frequency Division Multiplexing (OFDM) method is used in digital broadcasting or a communication system.
At this time, the OFDM transmission system inserts and transmits a pilot signal, which is used for frequency synchronization, symbol synchronization, and channel equalization in an OFDM reception system. Conventionally, since a channel equalizer of the OFDM reception system operates in a frequency domain, channel equalization performance deteriorates.
That is, the OFDM transmission system inserts and transmits the pilot signal in order to remove an effect of a multipath channel and the OFDM reception system performs channel equalization by using the pilot signal. When the same signal is received in different OFDM transmission systems and repeaters on a single frequency network (SFN), a 0 dB multipath signal is generated. When a channel is equalized by using the pilot signal in the frequency domain, performance deteriorates.
It will be described in detail hereinafter with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram showing an OFDM transmission system using a conventional pilot signal.
As shown in FIG. 1, the OFDM transmission system using the conventional pilot signal includes a channel encoder 10, a mapper 12, an Inverse Fast Fourier Transform (IFFT) 13, a guard interval inserter 14, and a Digital-to-Analog converter (DAC) 15.
The channel encoder 10 performs channel encoding on data to be transmitted including video, audio, and multimedia data. The mapper 12 performs mapping on the channel-encoded data of the channel encoder 10 and converts the data into an OFDM symbol. The channel encoder 10 inserts a pilot signal 11 to be used as a reference signal for channel equalization, frequency synchronization and time symbol synchronization in the OFDM reception system. The IFFT 13 performs IFFT on the OFDM symbol of the frequency domain, which is mapped in the mapper 12 and includes the channel-encoded data and the pilot signal, and transforms the OFDM symbol of the frequency domain into an OFDM symbol of the time domain. The guard interval inserter 14 inserts a guard interval for each symbol of the OFDM symbols transformed into the data of the time domain by the IFFT 13. The DAC 15 converts the digital OFDM symbol into an analog OFDM signal after inserting the guard interval by the guard interval inserter 14 in order to transmit the OFDM symbol to the OFDM reception system.
To have a look at an operation of the OFDM transmission system using the conventional pilot signal, data to be transmitted, which include video, audio, and multimedia data, pass through the channel encoder 10 to correct an error occurring in a channel.
The channel-encoded data are converted into an OFDM symbol in the mapper 12 for conversion. At this time, the OFDM reception system inserts the pilot signal 11 to be used as a reference signal for channel equalization, frequency synchronization and time symbol synchronization.
Subsequently, IFFT is performed on the OFDM symbol of the frequency domain including the channel-encoded data and the pilot in the IFFT 13 and the OFDM symbol of the frequency domain is transformed into the OFDM symbol of the time domain. The guard interval is inserted in each OFDM symbol converted into the time domain data in the guard interval inserter 14. After inserting the guard interval, the digital OFDM symbol is converted into the analog OFDM signal through the DAC 15 and transmitted to the OFDM reception system.
FIG. 2 is a block diagram showing an OFDM reception system using the conventional pilot signal.
As shown in FIG. 2, the OFDM reception system using the conventional pilot signal includes an analog-to-digital converter (ADC) 20, a guard interval remover 21, an FFT 22, a pilot signal extractor 23, a channel equalizer 24, a demapper 25, and a channel decoder 26.
The ADC 20 converts a received analog OFDM signal into a digital OFDM symbol of a time domain. The guard interval remover 21 removes a guard interval from the digital OFDM symbol of the time domain converted in the ADC 20. The FFT 22 transforms the OFDM symbol of the time domain into an OFDM signal of the frequency domain through FFT after removing the guard interval in the guard interval remover 21. The pilot signal extractor 23 extracts a pilot signal from the OFDM signal of the frequency domain transformed in the FFT 22.
The channel equalizer 24 performs channel equalization on the OFDM signal of the frequency domain from the FFT 22 by using the pilot signal extracted by the pilot signal extractor 23 in order to remove an effect of the channel. The demapper 25 performs demapping on the OFDM signal of the frequency domain after channel equalization in the channel equalizer 24 in order to acquire channel-encoded data transmitted from the transmission system. The channel decoder 26 performs channel decoding on the OFDM signal of the frequency domain, which is demapped in the demapper 25, in order to extract the data to be transmitted in the transmission system.
An operation of the OFDM reception system using the conventional pilot signal will be described hereafter. A received analog OFDM signal is converted into a digital OFDM symbol of a time domain through the ADC 20. Subsequently, the guard interval is removed by the guard interval remover 21 in the converted digital OFDM symbol of the time domain. After removing the guard interval, the OFDM symbol of the time domain is transformed into the OFDM signal of the frequency domain through FFT in the FFT 22. The pilot signal extractor 23 extracts a pilot signal from the transformed OFDM signal of the frequency domain.
The channel equalizer 24 performs channel equalization on the OFDM signal of the frequency domain from the FFT 22 based on the extracted pilot signal in order to remove an effect of the channel. The channel-equalized OFDM signal of the frequency domain is demapped by the demapper 25 and outputted as channel-encoded data transmitted from the transmission system. Channel decoding is performed on the demapped and channel-encoded data in the channel decoder 26 and the data are extracted as the data to be transmitted in the transmission system, which include video, audio, and multimedia data.
As described above, to remove the effect of the multipath channel, the OFDM transmission system inserts and transmits the pilot signal and the OFDM reception system performs channel equalization by using the pilot signal. In a single frequency network (SFN), since different OFDM transmission systems and repeaters transmit the same signal, there is a case when the OFDM reception system receives a deteriorated signal such as a 0 dB multipath signal. In this case, channel equalization is performed by using the pilot signal in the frequency domain and fading occurs causing deep null. There is a problem when channel equalization using the pilot signal in the frequency domain causes performance deterioration.